Non-aqueous secondary batteries (hereinafter occasionally simply referred to as “secondary batteries”) such as lithium ion secondary batteries are small and light, high in energy density, and capable of repeated cycles of charge and discharge. For such characteristics, secondary batteries are used in a wide variety of applications. A secondary battery generally includes non-aqueous secondary battery members such as a positive electrode, a negative electrode, and a separator for separating the positive electrode and negative electrode from each other to prevent a short-circuit between them.
In recent years, secondary battery members have been used that include a substrate for non-aqueous secondary battery, and a functional layer (e.g., a porous membrane layer for improving heat resistance and strength, or an adhesive layer for improving adhesion between secondary battery members) formed on the substrate using a slurry composition for non-aqueous secondary battery functional layer (hereinafter occasionally simply referred to as a “functional layer composition”) containing a binder component and a dispersion medium such as water.
Specific examples of secondary battery members that have been used in the art are electrodes that include an electrode substrate, which has an electrode mixed material layer provided on a current collector, and a functional layer formed on the electrode substrate; and separators that include a separator substrate and a functional layer formed on the separator substrate (see, e.g., PTLS 1 and 2).
PTL 1 discloses a porous membrane layer to be formed on an electrode substrate or separator substrate and reports that a porous membrane layer that comprises a binder (made of polymer particles having heterogeneous phase structure consisting of a core layer composed of a polymer of a vinyl monomer component, and of a shell layer composed of a polymer of a hydrophilic functional group-containing monomer component) and non-conductive particles exhibits superior strength as well as exerts high lithium ion diffusivity in lithium ion secondary batteries.
PTL 2 reports that a separator that includes a separator substrate, a porous membrane layer containing non-conductive particles and binder formed on the separator substrate, and an adhesive layer containing a particulate polymer having a specific glass-transition temperature formed on the porous membrane layer exhibits good adhesion to electrodes allowing a secondary battery to exert superior high-temperature cycle characteristics and rate characteristics.